The invention relates to a device for cutting a paper web in a transverse direction, comprising a continuously driven first conveyor, by means of which the paper web is moved at a constant first conveying speed into the device and is there stored in the form of a loop during the cutting operation, comprising a stationarily arranged cutting device, comprising a clamping device for the intermittent clamping of the paper web, and comprising a second conveyor to move the paper web after its storage at a second conveying speed increased in comparison to the first conveying speed, consisting of a continuously driven transport roller and several pressure rollers which can be intermittently pressed on the transport roller with the interpositioning of the paper web by means of a lifting device, whereby the clamping device and the lifting device can be driven in step with the cutting device in such a manner that for the duration of the cutting operation, the paper web is clamped by the clamping device, and the pressure rollers are lifted off from the transport roller.
The second conveyor is, in a conventional device for the transverse cutting of a paper web of this type (DE 31 31 101 A1), arranged in a feed direction of the paper web in front of the clamping device and the latter is placed directly in front of the cutting device. A joint crank drive is provided to operate the clamping device, the cutting device and the lifting device, which joint crank drive coordinates through three connecting rods the movements of the aforementioned devices in such a manner that for the duration of the cutting operation the paper web is clamped by the clamping device and the pressure rollers are lifted off from the transport roller. Since during the clamping of the paper web the first conveyor continuously runs and the second conveyor is inactive because of the lifted-off pressure rollers, a loop is formed in the area in front of the second conveyor. As soon as the cutting operation has ended, the pressure rollers are again pressed on the transport roller of the second conveyor and the paper web is moved on by said second conveyor at a conveying speed, which is increased relative to the first conveying speed, which first causes the loop to be undone and the paper web to be tightened. The paper web is subsequently moved on by the first conveyor and the second conveyor. The pressure rollers of the second conveyor are pressed by spring force on the transport roller, whereby this spring force is adjusted so weak that, with the paper web being tightened, slip occurs between the transport roller rotating at a high speed and the paper web. The conveying speed of the web is determined exclusively by the conveying speed of the first conveyor. In order for the mentioned slip between the transport roller of the second conveyor and the paper web to be able to occur, the second transport roller is designed as a metal roller. The maximum possible pulling force is hereby limited on the one hand in such a manner that no pressure points whatsoever may show in the paper web when the mentioned slip occurs, and on the other hand the paper web at the moment of the tightening of the same, when the loop has just been undone, may not tear. The second conveyor has thus a relatively small pulling force and therefore cannot sufficiently quickly accelerate the paper web from standstill, after it has been released from the clamping device, thus causing the cutting performance of this known device to be very limited. Furthermore, there also exists the danger of a paper jam. Namely, in order to achieve an as high as possible cutting performance, the pressure rollers of the second conveyor must, after the cutting operation has ended, be pressed as quickly as possible again on the transport roller. When at this time the clamping jaws of the clamping device are not yet completely opened, there exists the danger that the paper web gets caught on the partially open clamping jaws and a paper jam occurs. The pressure rollers can therefore only be pressed on the transport roller when the complete opening of the clamping device has been secured. This also results in limiting the cutting performance.
Another conventional device for the transverse cutting of a paper web (DE 196 24 277) has a design similar to the aforedescribed device, with the difference that the second conveyor is arranged between the clamping device and the cutting device, and that this second conveyor lacks a lifting device for lifting off the pressure rollers. The pressure rollers are in this conventional device continuously pressed by spring force on the continuously driven transport roller of the second conveyor. Since same is arranged in paper running direction behind the clamping device, however, the paper web is clamped by the clamping device during the cutting operation, a slip between the second conveyor and the paper web must be possible during the cutting operation. In order to accomplish this, the transport roller of the second conveyor is designed as a metal roller and the spring force acting on the pressure rollers is adjusted relatively low. The maximum possible pulling force of the second conveyor is here also limited in such a manner that no pressure points whatsoever may show in the paper web when the mentioned slip occurs and the paper web, at the moment of a tightening of the same, when the loop has just been undone, may not tear. Thus the second conveyor has here also a relatively small pulling force and can therefore not sufficiently quickly accelerate the paper web from standstill, after it has been released from the clamping device. The cutting performance also of this conventional device is limited in this manner at a form length of approximately 9 cm to 50,000 cuts per hour.
The basic purpose of the invention is therefore to provide a device for the transverse cutting of a paper web of the above-identified type, which most of all enables a significant increase in the cutting performance.
This is attained according to the invention in such a manner that the second conveyor is arranged between the clamping device and the cutting device, that the transport roller of the second conveyor has a surface coating with an as large as possible coefficient of friction relative to the paper web, and that the lifting device is designed such or can be controlled such that the pressure rollers are lifted off from the transport roller just prior to the paper web being tightened through a taking up of the loop.
The pressure rollers of the second conveyor can in this new device for the transverse cutting of a paper web be pressed immediately following the end of the cutting operation again on the transport roller of the second conveyor. As soon as this has taken place, the initially resting paper web is much accelerated by the transport roller. This high acceleration is possible because the surface coating of the transport roller has a high coefficient of friction relative to the paper web and can therefore transmit a high pulling force to same. The paper web can be accelerated already at a time, at which the clamping device is only partially open since the second conveyor is arranged behind the clamping device and therefore pulls the paper web through the clamping device. The danger of a paper jam at the clamping device is thus eliminated. It is also important that the pressure rollers are again lifted off from the transport roller by the lifting device shortly before the paper loop is undone and the paper web is tightened. Otherwise the paper web would namely tear due to the high acceleration of the paper web and the high pulling force of the transport roller. As a whole the time needed for an undoing of the loop is significantly shortened by the high acceleration of the paper web, which in particular in the case of short form lengths results in a significant increase of the cutting performance. Thus it is, for example, possible at a form length of 10 cm to achieve a cutting performance of up to 100,000 cuts per hour or more than 27 cuts per second. This means that within a time period of only 0.036 seconds the paper web is accelerated from standstill after the end of a cutting operation, is moved 10 cm by the second conveyor, is clamped by means of a clamping device, and is cut by the cutting device. At a form length of approximately 30 cm, the new device makes it still possible to reach a cutting performance of up to approximately 36,000 cuts per hour.